If Earth and the asteroid were travelling around a clock face, with the Sun in the middle, the asteroid runs about two numbers ahead.

However, the asteroid sometimes ranges so far ahead that it is on the opposite side of the Sun from Earth, said Martin Connors, of Canada's Athabasca University in Alberta. Mr Connors reports the work with colleagues in the latest issue of the journal Nature.

Asteroids are giant space rocks that orbit the Sun, and ones that share an orbit with a planet are called Trojans.

Scientists had previously found a few for Mars and Neptune and nearly 5,000 for Jupiter.

Spotting one in Earth's orbit is difficult from the ground because the potential locations are generally in the daytime sky.

The newfound object, called 2010 TK7, was discovered last year by Nasa's Wise satellite.

Mr Connors and colleagues were able to focus a ground-based telescope in Hawaii on it in April, determining its orbit with enough precision to show it was a Trojan.

Donald K Yeomans, manager of Nasa's Near-Earth Object Programme Office, who did not participate in the discovery, agreed that the asteroid is a Trojan. He said that most scientists suspected Earth had them, adding: "I would guess there's others."